tag->metricIDs
This should reduce the amounts of used RAM during queries with filters over big number of time series.
lib/uint64set
map[uint64]struct{}
This should improve inverted index search performance for filters matching big number of time series, since `lib/uint64set.Set` is faster than `map[uint64]struct{}` for both `Add` and `Has` calls. See the corresponding benchmarks in `lib/uint64set`.
make fmt
